1. Field of the Invention
The present invention relates to a system to predict residual settlement quantity and residual horizontal deformation quantity of ground where a liquefaction phenomenon occurred due to earthquake.
2. Prior Art
Liquefaction of ground is a phenomenon where cyclic shear stress due to earthquake motion increases pore water pressure in saturated sandy ground and reduces shear rigidity of ground. It is the phenomenon where settlement of ground or buildings, floating of light-weight underground structure, damage of piles due to a lateral flow phenomenon, is occurred by reduction of ground rigidity. Because random settlement of buildings or enormous damage occurs to infrastructure facility commencing with lifeline if the liquefaction phenomenon occurs, risk of the liquefaction phenomenon occurrence is predicted before constructing facility, and measures are taken for liquefaction if necessary.
Conventionally, there has existed a method of predicting the liquefaction phenomenon, in which volume of shear stress that would occur during earthquake is computed (computing external force), undrained cyclic shear strength of ground is found (computing resistance force), the ratio of the both forces is found as a liquefaction safety factor FL, and it is determined that the liquefaction phenomenon should occur when the value is 1 or less. However, this method is a bipolar prediction method predicting either the liquefaction phenomenon occurs or not, and is not a method that can explain a real phenomenon where damage quantity continuously and relatively differs depending on a magnitude relationship between the size of external force and the size of resistance. Particularly, a method capable of predicting an extent of damage by liquefaction is demanded regarding a structure requiring performance design.
It is possible to compute disaster quantity of soil structure or ground with occurrence of the liquefaction phenomenon during earthquake by the residual deformation quantity after the earthquake ends. Presently, as a method of predicting disaster deformation quantity, there exists a numerical analysis method where soil behavior is ideally modeled. The prediction accuracy of this method largely depends on modeling of the cyclic shear behavior of soil and on how the shear deformation behavior of soil in situ that would suffer earthquake can be truly modeled. However, universalization of the behavior of soil is not sufficient by the current technical level, and the complexity of the behavior of soil in situ has not been fully expressed.
Further, there has existed a problem that the conventional numerical analysis incorporated large uncertainty in modeling the cyclic shear behavior of soil. In particular, the behavior of soil after it suffered shear stress and its strain level became large is extremely important because it directly contributes to the residual deformation quantity of ground, but no ultimate solution exists in modeling this area.
In addition, the conventional numerical analysis method has paid attention solely to undrained shear behavior of soil, where it is assumed that interstitial water in soil does not move during vibration, and in most cases, only deformation during vibration is considered regarding the deformation. However, it has been made clear with eyewitness evidence of actual earthquake damage that the residual deformation quantity of ground during earthquake does not only occur during vibration but it also develops progressively after vibration, and therefore, a method of examining mechanism of deformation development after vibration and predicting deformation based on the mechanism is demanded.
FIG. 1 is an appearance where the liquefaction phenomenon occurs due to earthquake. When earthquake begins, in FIG. 1, excessive pore water pressure occurs due to cyclic shear. Although the excessive pore water pressure due to earthquake ceases after earthquake ends, deformation of ground caused by inflow and outflow of interstitial water occurs with the cease. The deformation of ground continues until the excessive pore water pressure resolves.
Accordingly, a possibility has been pointed out recently, that seepage of the excessive pore water pressure plays an important role in flow deformation mechanism after earthquake (Motoki Kazama et al., Flow deformation mechanism due to seepage failure after earthquake, The 36th Geotechnical Engineering Symposium, pp.2415-2416, 2001). Furthermore, it has been found out that a preliminary numerical analysis, where bulk compression characteristic when the excessive pore water pressure caused by the cyclic shear resolves (relationship between interstitial water drainage and recovery of effective stress) and shear deformation characteristic when interstitial water flows in (relationship between interstitial water inflow and shear strain) are assumed, can explain deformation quantity after earthquake naturally (Noriaki Sento et al., Prediction method of flow deformation due to seepage failure after earthquake, The 36th Geotechnical Engineering Symposium, pp.2417-2418, 2001).
Few studies have been made for deformation characteristic of soil at the point of the inflow and outflow of such interstitial water, and the inflow and outflow of interstitial water is determined by environmental boundary conditions, so that damage prediction has been limited only by a characteristic prehension test as an element.
The object of the present invention is to provide a system to accurately predict residual settlement quantity and residual horizontal deformation quantity of ground, where the liquefaction phenomenon occurred due to earthquake, after resolving excessive pore water pressure.
To achieve the above-described object, the present invention is a liquefaction phenomenon prediction system that comprises shear stress loading means that applies shear stress to a test piece from subject ground, interstitial water injecting means that gives interstitial water to the test piece, measuring means that measures displacement and a pore water pressure at certain time, and data collection/control means that collects data from the measuring means and controls the shear stress loading means and the interstitial water injecting means, in which the data collection/control means controls the shear stress loading means to apply earthquake motion to the test piece, performs vibration analysis and seepage flow analysis for the test piece every hour by data from the measuring means, controls the shear stress loading means and the interstitial water injecting means based on a result obtained, and thus finds final residual deformation quantity.
The vibration analysis and the seepage flow analysis in the data collection/control means can be performed by using the fact that, when the subject ground is spatially discretised into each stratum, each stratum satisfies a force balance equation regarding shear stress and a continuity equation of balance regarding the movement of interstitial water every hour.
The data collection/control means performs numerical analysis of a stratum where the characteristic of the subject ground is known, and a numerical analysis result can be used in the vibration analysis and seepage flow analysis for the stratum.